Over the past decade, there has been an increased interest within the polyurethane industry to use natural oil based polyols, either as stand-alone products or in conjunction with petroleum based polyols. The two main reasons for this growing interest are: heightened awareness of “green” issues that can minimize greenhouse gas emissions as well as reducing the dependency on non-renewable sources.
The use of biomass as raw materials for production of fuels and chemicals to displace fossil resources has been the focus of many research activities. These activities are motivated by the possibility of positive contributions to a sustainable resource supply, enhanced national security, and macroeconomic benefits for local communities and society at large.
Many of these activities were directed toward fuel and energy production (e.g. fermentation of biomass to ethanol and trans esterification of triglyceride oils to biodiesel).
Only limited effort has been directed toward value added industrial products using the protein biomass that is left behind after extracting the oil. The amino acids in the protein are rich in amine, functional groups that can easily be converted to urethanes.
Many such products can be derived from such polyurethanes and include foams, coatings, adhesives, sealants and elastomers, among many others. Currently, only a few of these synthetic polyurethanes contain bio-based components and these are generally prepared by condensation of petroleum-based isocyanates with vegetable oil based polyols.
Furthermore, it is well known that soybean contains much more protein than oil (see FIG. 1). Although the exact composition of the bean depends on many variables including trait, climax, soil, geographical location, maturity, the extraction process, etc.
Typically, the protein content is almost twice the content of the oil (about 38% compared with only 18% oil). Furthermore, the cost of the protein meal is about half the cost of the oil. Thus, the use of the meal as a raw material is attractive considering the economics and its availability. In the United States, soybeans are readily available and are grown in large quantities. After extracting the oil, most of the soy meal is processed toward animal feed, primarily for poultry, swine, cattle, and aquaculture.
A very small portion is refined to soy flour, soy concentrates, and soy isolates for human consumption and only 0.5% is used for industrial applications. The small amounts of meal used for industrial applications include its use as adhesives for plywood and particle board with other minor applications such as additives in textured paints, insecticides, dry-wall tape compounds, linoleum backing, paper coatings, fire-fighting foams, fire-resistant coatings, asphalt emulsions, cosmetics, and printing inks.
Currently, essentially all polyurethanes used in the industry are made by the phosgenation of amines and then reacting the so-produced isocyanates with polyols. The use of phosgene is economical but it is hazardous and can only be done using special equipment due to the extreme toxicity of phosgene and the high volume of HCl by-products. A number of phosgene-free methods for preparing isocyanates have been reported in the literature.
Patent application EP-A 583.637 discloses the decomposition of tri-substituted ureas at elevated temperature (90-400° C.) and in the presence of a solvent, into a volatile mono-isocyanate and a secondary amine of which the boiling point is higher than that of the isocyanate and higher than the reaction temperature.
U.S. Pat. No. 3,936,484 describes the decomposition of tri-substituted ureas at elevated temperatures (above 230° C.) and in the presence of an inert carrier to form isocyanates and amines. The reported isocyanate yield by this method was relatively low from 60 to 88%.
French patent application A 1473821 describes the pyrolysis of substituted ureas in the Liquid phase (temperature less than 200° C.) in the presence of high boiling solvents, into isocyanates and amines. The reaction times, however, by this method are long (6-35 hours) and the isocyanate yield is only a moderate (60-75%).
Several other methods are known (European patent applications EP-A 391716, EPA 402020 and EPA 408277) whereby isocyanates are prepared by thermally decomposing dialkylureas in an inert solvent in the presence of co-reagents.
Another method is based on the interaction of alkyl ethers of sulfuric or phosphoric acid with cyanates of metals. The reported yield of the isocyanate products is as high as 90% by weight. However, the starting ethers are rarely available and, in some cases, toxic compounds.
Preparation of isocyanates by way of catalytic carbonylation of nitroalkyl compounds requires high temperature within the range of from 100 to 250° C. and pressures of from 100 to 500 atm. The method of preparing isocyanates (with a yield of from 42 to 73%) by decomposition of N-formamides in the presence of chlorine-containing agents has not obtained practical application due to the multi-stage character of the process.
Among phosgene-free methods the Kurcius method is only a laboratory method due to the risk of explosion upon heating of inorganic and organic azides. Also known is preparation of a mono-isocyanate by pyrolysis of esters of carbamic acid in the presence of P2O5 at a high temperature (100 to 500° C.). However, only low yields were reported by this process.
U.S. Pat. No. 4,192,815 discloses the reaction of primary amines with carbon dioxide and hexamethyldisilazane in the presence of an acidic catalyst to yield silyl esters of carbamic acid, which are then decomposed in the presence of dehydration agents at elevated temperatures. Unfortunately, these esters are decomposed to a mixture of siloxanes, trimethylchlorosilane and mono-isocyanates and must be further purified.
It is apparent from this brief introduction that these alternative methods are limited. They either need to be carried out in dilute solutions using expensive high boiling solvents or require the use of co-reagents and the production of considerable amounts of undesirable by-products. The process used herein is environmentally friendly and does not require any special equipment or novel materials. Furthermore, it leads to aliphatic polyurethanes that are especially suitable for applications that require abrasion resistance and stability against, degradation from UV. These properties are particularly desirable for instance in the exterior paint and enamel coatings industry.